Hypericin Mediated Photodynamic Inactivation for the treatment of chronically infected Diabetic Foot Ulcers

Introduction
Diabetes-related foot ulcers (DFUs) are a major complication of diabetes, frequently progressing to infection and osteomyelitis, and associated with high morbidity, mortality, and healthcare costs. Rising antimicrobial resistance further limits therapeutic options. This thesis combines a retrospective observational study of the microbiology of diabetic foot osteomyelitis (DFO) with laboratory evaluation of antimicrobial photodynamic inactivation (aPDI) as a novel non-antibiotic therapeutic strategy.

Methods
Bone specimens taken from patients with DFO attending the Diabetic Limb Salvage Clinic at Leeds Teaching Hospitals NHS Trust between 2017 and 2020 were analysed retrospectively for microbiological and resistance profiles. Resistance was classified using international definitions of multidrug-resistant organisms (MDROs). Findings from this clinical study informed the design of subsequent in vitro experiments evaluating the efficacy of aPDI against S. aureus. Hypericin was used as a photosensitiser in combination with controlled light exposure, and a hydrogel formulation was developed to improve drug delivery and applicability with the ultimate aim of developing a smart plaster system.

Results
A total of 251 patients were included, contributing 322 ulcers and 392 bone specimens. The microbiological profile was heterogeneous, with Staphylococcus spp., Enterobacterales, Enterococcus spp., and Pseudomonas frequently isolated. Resistance to commonly used antibiotics was widespread, and 13.5% of patients harboured a Multidrug-Resistant Organism (MDRO). Based on these findings, S. aureus was selected as a suitable gram-positive organism for in vitro testing. APDI achieved significant reductions in bacterial viability, and incorporation of hypericin in a hydrogel system supported controlled release suitable for wound application.

Conclusion
This thesis provides new insights into the microbiology and resistance patterns of DFO in local area and demonstrates proof-of-concept evidence that hydrogel-mediated aPDI is effective against S. aureus. Together, these findings highlight the limitations of conventional antibiotic therapy and support further development of aPDI as a novel adjunctive treatment for diabetic foot infections.